1. Field of the Invention
The present invention relates to an ultrasonic imaging diagnosis apparatus which can provide tissue images, blood flow rate data and blood-flow images of human bodies under examination.
2. Description of the Related Art
To make a diagnosis of circulatory organs, it is important to measure a blood flow rate, to observe the motion of the cardiac muscle and the flow of the blood particularly a cardiac output indicating blood flow discharged from the heart to the whole body via the aorta. Conventionally various measuring methods have been proposed. The ultrasonic imaging method is the best of those used, because it is noninvasive and simple. With this method, ultrasonic beams are emitted in multiple directions, the blood flow velocity is obtained from phase information of reflected waves, and the blood flow rate is obtained from the blood flow velocity.
However, the blood flow measurement method using ultrasonic waves has two factors that lower the measurement accuracy. Consider the case of measurement of a cardiac output. Suppose that a sector scan which is suitable for examination of hearts is used.
First, the first factor will be described. The doctor puts an ultrasonic probe to the chest wall of a human body under examination and changes the angle and position of the probe relative to the chest wall while watching a tomography image in real time so as to fit the scanning plane of the probe to an optimum sectional plane taken through the lengthwise dimension of the outflow tract of the heart. The tomography image along this scanning plane is displayed in real time on a TV monitor. The doctor then operates a mouse or trackball to set up a line of interest or a region of interest (hereinafter referred to simply as a line of interest) in an optimum position on the tomography image so that it will cross the outflow tract. Finally, the doctor instructs the CPU in the imaging apparatus to start calculation of the blood flow rate.
By the way, the angle and position that the probe takes relative to the chest wall must be adjusted with high accuracy. This is because even if the angle and position of the probe are changed slightly, the scanning plane will deviate from the optimum sectional plane. Thus, the doctor must concentrate on adjusting the probe so that its scanning plane will not deviate from the optimum sectional plane until the measurement is terminated. Under such condition, it is very difficult for the doctor to perform the operation of setting the line of interest. If the scanning plane deviates from the optimum sectional plane, the line of interest cannot be placed accurately in the desired position, which will result in reduced accuracy of blood flow measurement.
The second factor will be described next. A two-dimensional scan in the Doppler mode for blood flow measurement is made as follows. First, the transmission/reception of ultrasonic waves is performed for a first scanning line. The transmission/reception of ultrasonic waves for the first scanning line is repeated at least twice, normally 16 times. After the transmission/reception of ultrasonic waves is successively repeated 16 times for the first scanning line, the same operation is performed for the next scanning line. A one-frame scan is completed by repeating the transmission of ultrasonic waves 16 times for the same scanning line and shifting the ultrasound transmission path from each scanning line to the next adjacent scanning line.
Therefore, there is some difference in actual data acquisition time between each scanning line and the next adjacent scanning line. Suppose, for example, that the repetition period is 200 .mu.s, the number of times the transmission of ultrasonic waves is repeated for the same scanning line is 16, and the number of scanning lines for one frame is eight. Then, a time difference between two adjacent scanning lines will be EQU 200 .mu.s.times.16=3.2 ms
The time difference between the first transmission of the first scanning line and the sixteenth transmission of the last scanning line in one scan will be EQU 200 .mu.s.times.16.times.8=25.6 ms
The blood flow rate is given by integrating the flow velocity at points on the cross section of a blood vessel. Thus, the time difference between scanning lines will produce an error in the blood flow rate instantaneously obtained. That is, the blood flow rate will be measured as an integration value of flow velocities measured at different times, which results in a reduction in measurement accuracy. Further, the time difference will make it somewhat difficult to observe the motion of the cardiac muscle and the flow of the blood.